/*
 * Copyright (C) 2012 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.base;

import com.google.common.annotations.GwtIncompatible;

import javax.annotation.CheckForNull;

/**
 * Represents a {@linkplain System#getProperties() standard system property}.
 *
 * @author Kurt Alfred Kluever
 * @since 15.0
 */
@GwtIncompatible // java.lang.System#getProperty
@ElementTypesAreNonnullByDefault
public enum StandardSystemProperty
{

    /**
     * Java Runtime Environment version.
     */
    JAVA_VERSION("java.version"),

    /**
     * Java Runtime Environment vendor.
     */
    JAVA_VENDOR("java.vendor"),

    /**
     * Java vendor URL.
     */
    JAVA_VENDOR_URL("java.vendor.url"),

    /**
     * Java installation directory.
     */
    JAVA_HOME("java.home"),

    /**
     * Java Virtual Machine specification version.
     */
    JAVA_VM_SPECIFICATION_VERSION("java.vm.specification.version"),

    /**
     * Java Virtual Machine specification vendor.
     */
    JAVA_VM_SPECIFICATION_VENDOR("java.vm.specification.vendor"),

    /**
     * Java Virtual Machine specification name.
     */
    JAVA_VM_SPECIFICATION_NAME("java.vm.specification.name"),

    /**
     * Java Virtual Machine implementation version.
     */
    JAVA_VM_VERSION("java.vm.version"),

    /**
     * Java Virtual Machine implementation vendor.
     */
    JAVA_VM_VENDOR("java.vm.vendor"),

    /**
     * Java Virtual Machine implementation name.
     */
    JAVA_VM_NAME("java.vm.name"),

    /**
     * Java Runtime Environment specification version.
     */
    JAVA_SPECIFICATION_VERSION("java.specification.version"),

    /**
     * Java Runtime Environment specification vendor.
     */
    JAVA_SPECIFICATION_VENDOR("java.specification.vendor"),

    /**
     * Java Runtime Environment specification name.
     */
    JAVA_SPECIFICATION_NAME("java.specification.name"),

    /**
     * Java class format version number.
     */
    JAVA_CLASS_VERSION("java.class.version"),

    /**
     * Java class path.
     */
    JAVA_CLASS_PATH("java.class.path"),

    /**
     * List of paths to search when loading libraries.
     */
    JAVA_LIBRARY_PATH("java.library.path"),

    /**
     * Default temp file path.
     */
    JAVA_IO_TMPDIR("java.io.tmpdir"),

    /**
     * Name of JIT compiler to use.
     */
    JAVA_COMPILER("java.compiler"),

    /**
     * Path of extension directory or directories.
     *
     * @deprecated This property was <a
     * href="https://openjdk.java.net/jeps/220#Removed:-The-extension-mechanism">deprecated</a> in
     * Java 8 and removed in Java 9. We do not plan to remove this API from Guava, but if you are
     * using it, it is probably not doing what you want.
     */
    @Deprecated
    JAVA_EXT_DIRS("java.ext.dirs"),

    /**
     * Operating system name.
     */
    OS_NAME("os.name"),

    /**
     * Operating system architecture.
     */
    OS_ARCH("os.arch"),

    /**
     * Operating system version.
     */
    OS_VERSION("os.version"),

    /**
     * File separator ("/" on UNIX).
     */
    FILE_SEPARATOR("file.separator"),

    /**
     * Path separator (":" on UNIX).
     */
    PATH_SEPARATOR("path.separator"),

    /**
     * Line separator ("\n" on UNIX).
     */
    LINE_SEPARATOR("line.separator"),

    /**
     * User's account name.
     */
    USER_NAME("user.name"),

    /**
     * User's home directory.
     */
    USER_HOME("user.home"),

    /**
     * User's current working directory.
     */
    USER_DIR("user.dir");

    private final String key;

    StandardSystemProperty(String key)
    {
        this.key = key;
    }

    /**
     * Returns the key used to lookup this system property.
     */
    public String key()
    {
        return key;
    }

    /**
     * Returns the current value for this system property by delegating to {@link
     * System#getProperty(String)}.
     *
     * <p>The value returned by this method is non-null except in rare circumstances:
     *
     * <ul>
     *   <li>{@link #JAVA_EXT_DIRS} was deprecated in Java 8 and removed in Java 9. We have not
     *       confirmed whether it is available under older versions.
     *   <li>{@link #JAVA_COMPILER}, while still listed as required as of Java 15, is typically not
     *       available even under older version.
     *   <li>Any property may be cleared through APIs like {@link System#clearProperty}.
     *   <li>Unusual environments like GWT may have their own special handling of system properties.
     * </ul>
     *
     * <p>Note that {@code StandardSystemProperty} does not provide constants for more recently added
     * properties, including:
     *
     * <ul>
     *   <li>{@code java.vendor.version} (added in Java 11, listed as optional as of Java 13)
     *   <li>{@code jdk.module.*} (added in Java 9, optional)
     * </ul>
     */
    @CheckForNull
    public String value()
    {
        return System.getProperty(key);
    }

    /**
     * Returns a string representation of this system property.
     */
    @Override
    public String toString()
    {
        return key() + "=" + value();
    }
}
